Recently, there has been a demand for reduction in size (diameter) of a spark plug in order to attain a higher degree of engine design flexibility for improvement in engine performance, such as engine output and efficiency. For example, the diameter reduction of the spark plug leads to the formation of a smaller plug hole and permits the arrangement of a larger water jacket and intake/exhaust ports in the engine.
It is, however, undesirable to simply reduce the diameter of the spark plug because a crimp portion formed in an end portion of a metal shell for fitting the insulator to the metal shell is made small. As a result, a problem, such as air leakage and the ceramic insulator falling out from the metal shell, tends to occur (refer to Japanese Patent Application Laid-Open (kokai) No. 2007-258142).